1. Field of the Invention
The present invention relates generally to techniques for reducing power consumption of a mobile unit during periods of no-speech in a digital mobile telephone system. More specifically, the present invention relates to a method and apparatus for periodically energizing and de-energizing a decoder for generating background noise samples for power conservation, during periods of no-speech.
2. Description of the Related Art
It is known in the art that many efforts have been made to reduce power consumption of mobile units in a digital mobile telephone system. One such power conservation technique is to terminate radiation of data in the absence of a speech signal to be transmitted. That is, during periods of no-speech, the transmitter ceases signal radiation except for periodic transmission of background noise information. However, no proposal has been made for intermittently terminating the operation of a decoder, which is provided in a receiver, during the periods of no-speech.
Prior to turning to the present invention, it is advantageous to briefly describe a conventional technique for generating background noise samples using periodically transmitted background noise information. That is, this conventional technique is to fill in the no-speech periods using the noise information intermittently transmitted to the receiver. The signal processing in the digital telephone system is typically implemented on a frame-by-frame basis at both the transmitter and receiver. It is assumed, for a better understanding of the present invention, that each frame length is 10 ms and the frame of background noise information is transmitted at a time interval of one minute.
Referring to FIG. 1, a series of framed background noise information is periodically transmitted from a transmitter (not shown) during time periods of no-speech.
The transmitted framed sound signal (speech and no-speech) is received at an antenna 10 and is fed to a receiving section (RX) 12 wherein the transmitted signal is translated to an intermediate frequency (IF). Further, the receiving section 12 demodulates the incoming signal and outputs a baseband signal. This baseband signal is a compressed signal and thus it is necessary to be decompressed (expanded) before being applied to a digital-to-analog (D-A) converter whose output drives a speaker.
The compressed baseband signal takes the form of a series of frames each including 100 bits (for example). One bit of each frame is dedicated to an indicator, which specifies whether the frame is a speech signal or a no-speech signal. The speech/no-speech indicator bit is typically a leading bit of each frame. The output of the receiving section 12 is applied, on a frame-by-frame basis, 1 to a bit signal separator 14 which separates the one-bit of speech/no-speech indicator and the remaining bits (viz., 99 bits according to the above-mentioned assumption). The data bits are applied to a buffer 16, while the speech/no-speech indicator bit is applied to a buffer 18. Assuming that the speech/no-speech indicator bit takes a logic xe2x80x9c1xe2x80x9d for indicating that the corresponding frame is a speech frame while taking a logic xe2x80x9c0xe2x80x9d for indicating that the corresponding frame is an unvoiced frame.
A background noise data generation controller 20 responds to the output of the buffer 18 and controls a switch 22 as follows. That is, when the output of the buffer 18 is a logic xe2x80x9c1xe2x80x9d, the controller 20 controls the switch 22 so as to relay the speech signal (frame) to a decoder 26 via a terminal 22a. Thus, the decoder 26 decompresses (expands) the applied speech signal (digital) and applies the decompressed signal to a digital-to-analog (D-A) converter 28. The analog audio signal thus generated is applied, via a speaker driver 30, to a loudspeaker 32 at which an original sound is reproduced.
On the other hand, when the output of the buffer 18 takes a logic xe2x80x9c0xe2x80x9d, the controller 20 controls the switch 22 so as to relay one frame of background noise information to a memory 24 via a terminal 22b. Although it is not evident from FIG. 1, the background noise information (one frame) bypasses the memory 24 and at the same time is stored in the memory 24. Thereafter, until the next background noise information is received, the noise information stored in the memory 24 is read out memory on a frame-by-frame basis and is decompressed at the decoder 26. The decompressed noise signal is applied to the D-A converter as in the case of the speech signal.
It is understood that the decoder 26 continues to be energized irrespective of whether the incoming frame is the speech or no-speech signal.
Japanese Laid-open Patent Application No. 5-122165 discloses a background noise sample generating technique similar to the above. This Japanese Application teaches an intermittent transmission of background noise or parameters during the periods of no-speech. Further, the above-mentioned Japanese Patent Application discloses that the background noise information contains a noise parameter which is used to synthesize background noise data in order to reduce discomfort to a listener. However, the aforesaid Japanese Patent Application fails to teach or suggest intermittent de-energizing of a decoder for power conservation.
It is therefore an object of the present to provide techniques via which a decoder is intermittently de-energized during periods of no-speech thereby to implement power conservation of a mobile unit.
In brief, this object is achieved by techniques wherein the background noise data is generated using at least one noise parameter that is transmitted in a manner included in framed noise information. This information is transmitted at predetermined time intervals during a period of no-speech. A controller is provided so as to check to determine if an incoming framed data is the noise information. In the case where the incoming framed data is specified as the noise information, a check is made to determine if a time period, which corresponds to a predetermined number of consecutive frames, has expired. When the time period has not yet elapsed, the background noise data is generated using at least one noise parameter in a manner of extending to the predetermined number of frames. The background noise data thus generated is decoded at a decoder on a frame-by-frame basis so as to generate decoded background noise samples, and then these noise samples are applied to a digital-to-analog converter and simultaneously stored in a memory. After the background noise sample generation is finished, the decoder is de-energized. The decoded background noise samples already stored in the memory are successively retrieved and are converted into audible signal.
One aspect of the pre sent invention resides in a method of generating background noise samples in a radio receiver. The method comprises the following steps. A check is made to determine if an incoming framed data is noise information, after which a further check is made to determine if a time period corresponding to a predetermined number of frames has expired if the incoming framed data is specified as the noise information. The background noise data is generated, using the at least one noise parameter included in the incoming framed data, in a manner that the noise data extends to the predetermined number of frames if the time period has not expired. Then, a decoder is energized to successively decode the background noise data so as to generate decoded background noise samples that are then stored in a memory. The decoded background noise sample thus stored in the memory are read out thereof during a period of no-speech. The background noise samples are renewed using each of the following noise information intermittently transmitted.
Another aspect of the present invention resides in an apparatus for generating background noise samples at a radio receiver. The apparatus comprises, a controller for controlling generation of background noise data, the controller checking to determine if an incoming framed data is noise information. A background noise data generator is provided which, in response to a check result of the incoming framed data being the noise information, generates the background noise data using said at least one noise parameter included in the incoming framed data. The background noise data extends to a predetermined number of frames. A decoder decodes the background noise data, which has been generated by said background noise data generator, so as to generate background noise samples. A memory is provided for storing the decoded background noise samples. The decoded noise samples stored in the memory is retrieved during a period of no-speech while de-energizing the decoder.